This invention relates to circuitry to prevent excessive output current in composite transistors--useful, for example, in an audio-power amplifier to prevent overdissipation of an output transistor.
Over-current protection utilizing a sensing resistor in the emitter connection of a power amplifier output transistor is known. As the emitter current of the output transistor is increased in response to an increase in applied base current, a potential is developed across the resistor. By arranging to clamp the base potential of the output transistor when the potential across the resistor increases to such degree as indicates the onset of an over-current condition, the availability of base current to support additional emitter current flow is curtailed. So the power consumed by the transistor can be limited to a safe level without need for resorting to negative feedback circuits, which tend to be oscillatory under certain conditions. In the past, the potential to be developed across the sensing resistor in order that further increase of emitter current in the output transistor be forestalled has been a fixed potential. This has meant that the safe level of dissipation has had to be set lower than necessary for most operating conditions.